The invention provides a bioassay for detection of Mycobacterium tuberculosis infection comprising artificially modified peptide subsequences of the T-cell epitope from M. tuberculosis Ag85B. Particularly preferred peptides have the form: SGGNNSPAX (SEQ ID 26), where X is Methionine (SEQ ID 18), Leucine (SEQ ID 17), Alanine (SEQ ID 15) or Valine (SEQ ID 10) and NNSPAV (SEQ ID 14). The invention also provides peptides for use in such an assay.